Electronic devices are typically manufactured by producing multiple copies of the same device on a substrate or workpiece. In particular, semiconductor devices are manufactured on substrates referred to as wafers, which are thin disks of materials such as silicon, gallium arsenide or sapphire or other materials which are capable of supporting the various processes that create semiconductor devices. These devices at some point in the manufacturing process need to be separated into individual devices for subsequent packaging and use. This separation into individual devices is referred to as “singulation”. Singulation can be performed mechanically, using diamond-coated saw blades, chemically, by masking and etching, photonically by directing laser energy at the wafer or substrate, or combinations of these methods. Singulation can be accomplished by cutting completely through the wafer or substrate, or by making a partial cut or cuts into one or more surfaces of the wafer or substrate and then mechanically cleaving the wafer or substrate in to individual dice. Cutting completely through the wafer or substrate is commonly referred to as “dicing” and cutting partially through the wafer or substrate in preparation for subsequent cleaving is commonly called “scribing”. In general, devices are rectangular in shape and are laid out on a grid pattern on the wafer or substrate, allowing the devices to be separated by making a series of cuts either fully through the substrate (dicing) or partially through the substrate (scribing) between the devices in first in one direction 16 and then in a direction at 90 degrees to the first direction 14, as illustrated in FIG. 1. Referring to FIG. 1, singulation of devices on substrates generally proceeds by forming a series of cuts in one direction, for example the X direction between each row of devices then forming a series of cuts in the perpendicular Y direction between each column of devices thereby separating each device from the other devices on the wafer.
Issues related to device singulation include real estate, device damage, cost, complexity and system throughput. Real estate refers to the fact that in order to permit singulation without damage to devices, room must be left between the devices to allow room for cuts to be made without having to cut through active devices. The area between the active devices is known as a “street”. Streets 14, 16 are shown in FIG. 1 between active devices 12 on a wafer 10. Since the cost of processing a wafer or substrate is generally fixed, more devices per substrate yields greater profits for the manufacturer. This places a premium on making the streets as narrow as possible to squeeze as many active devices as possible onto the wafer or substrate. Factors working against narrow streets include the size of the kerf and the width of the damaged area next to the cut. While lasers typically can cut a narrower kerf than mechanical saws or chemical etch, lasers also typically create a heat affected zone (HAZ) next to the kerf due to the heat generated by the intense amount of energy required to cut the wafer or substrate. This HAZ should not be allowed to overlap the active device area or it may either cause part failures immediately or limit the useful life of the part. The HAZ also can cause device failure from cracks or chips in the HAZ propagating from the HAZ to the active device area of the device as the device is used.
Another problem related to laser singulation of wafers or substrates is the re-deposit of material removed from the kerf. The laser typically removes material from the substrate or wafer to form the kerf by both ablation and thermal means. The laser beam may be energetic enough to ablate material it impinges near the center of the beam, meaning that the material is ionized and forms a plasma cloud as it exits the kerf. Nearer the periphery of the beam or directly adjacent to the beam the material of the substrate may not receive enough energy to ablate the material but rather vaporizes or melts and boils the material. In this case the material is ejected from the kerf as it vaporizes or boils. As the material is ejected from the kerf and moves away from the laser beam it cools and re-solidifies as debris near the edges of the kerf. If this debris reaches active circuit areas of the device it can cause undesirable electronic malfunctions and therefore must be removed prior to packaging the device.
One means for predicting the reliability of semiconductor die which have been diced or scribed from a wafer is to test die break strength. In this test, a semiconductor die is subjected to bending to the point of failure. Variations is die break strength can predict semiconductor component failures, therefore, wafer dicing or scribing methods that improve die break strength also improve component reliability. Things that are known to reduce die break strength include chips or cracks along the edge of a cut caused by mechanical or laser cutting. In addition, debris re-deposited along the edge of the cut from materials vaporized or liquefied by the laser can cause damage to the circuit and reduce reliability.
In particular, laser parameters that provide desired cutting speed and kerf size and shape also cause a HAZ at the top edge of the cut and also tend to create debris from material removed the kerf which is re-deposited at the edge of the cut. FIG. 2 shows a cross-sectional view of a silicon wafer that has been cut with a laser showing the heat affected zone and debris caused by redeposit of material from the kerf. FIG. 2 shows a cross-sectional view of a cut wafer 20, having a top surface 22 and a bottom surface 24 and die attach film (DAF) 26. One side of the kerf 28 is shown. Also shown are re-deposited debris 30 and the HAZ 32. The HAZ can cause a reduction in die break strength and the re-deposited debris can cause device failure. An exemplary laser processing device for performing this singulation is the ESI Cignis Laser Singulation machine, manufactured by Electro Scientific Industries, Inc., Portland Oreg., 97229. This machine uses a picosecond laser to singulate silicon and other substrate materials.
Another issue with laser singulation of substrates or wafers is improving system throughput. In particular, laser parameters which provide higher cutting speed also create more HAZ and re-deposited debris, which is undesirable. It is also noted that laser parameters which provide high cutting speed also create a debris cloud at the cutting site. This debris cloud is comprised of plasma created by the laser pulses and gaseous, liquid and solid debris created by the laser pulses and ejected from the workpiece. It is known that the debris cloud created by laser pulses can absorb energy from subsequent laser pulses. FIG. 3 is a graph showing depth of cut vs. number of pulses for an exemplary laser substrate cutting process. This shows that cutting a silicon workpiece with a 4 Watt Nd: YVO4 laser with 10 nanosecond pulses aimed at a single point at a repetition rate of 5 KHz. As can be seen in FIG. 3, in this example cutting saturates at about 10 pulses. This saturation is believed to be caused by the debris cloud created by the first laser pulses. A debris cloud created in this manner not only blocks laser radiation from reaching the workpiece and prevents material removal it also absorbs laser energy heating the plasma further. As the plasma absorbs energy it heats up and due to its proximity to the workpiece transfers some of that heat energy to portions of the workpiece including sidewalls of the feature being machined. In addition vaporous, liquid or solid material in the cloud may be ejected from the cloud and deposited on the workpiece. This transferred energy and material causes cracking, deterioration of the feature sidewall and increased debris. In addition, if more energetic pulses or more pulses are directed to the workpiece in an effort to transmit more energy through the debris cloud to the workpiece and thereby continue to machine material, more energy is coupled into the debris cloud making the cracking, deterioration and debris problems worse. Even ultra fast processes which use short duration pulses in the picosecond or femtosecond range to ablate material before the material has time to transfer heat to adjacent regions cannot avoid coupling energy into the debris cloud. This energy causes consequent damage to the workpiece depending upon the material and the laser parameters used.
Issues related to dicing or scribing wafers or substrates have been the subject of previous work. U.S. Pat. No. 6,271,102 METHOD AND SYSTEM FOR DICING WAFERS, AND SEMICONDUCTOR STRUCTURES INCORPORATING THE PRODUCTS THEREOF, inventors Donald W. Brouillette, Robert F. Cook, Thomas G. Ference, Wayne J. Howell, Eric G. Liniger, and Ronald L. Mendelson, Aug. 7, 2001, describes cutting wafers with chamfered edges with a saw blade prior to laser cutting from both the front and backside to improve die break strength. In this case the chamfered cuts are produced instead of straight cuts with either a laser or a saw. U.S. Pat. No. 7,129,114 METHODS RELATING TO SINGULATING SEMICONDUCTOR WAFERS AND WAFER SCALE ASSEMBLIES, inventor Salman Akram, Oct. 31, 2006, tries to solve the problem of die break strength by scribing trenches next to the laser cut and coating the trenches with protective material. US patent publication No. 2006/0249480, LASER MACHINING USING AN ACTIVE ASSIST GAS, inventor Adrian Boyle, Nov. 9, 2006, discloses using an assist gas to etch away the damaged portion of the edges of the cuts to improve die break strength. WIPO patent publication No. 2008/064863 LASER MACHINING, inventors Kali Dunne and Fallon O'Briain, Jun. 5, 2008 discusses using particular patterns of laser pulse spacing along the path being cut with multiple passes to avoid the debris cloud.
What these references have in common is a desire to improve die quality following dicing or scribing by attempting to overcome problems with die break strength and debris caused by laser cutting. These methods either require that cutting be done from both the top and the bottom of a wafer, be accomplished by a combination of laser and mechanical sawing, or require additional processing steps and equipment such as chemical etching, or require multiple passes by the laser. What is needed then is an efficient way to dice wafers with a laser that does not require additional processing steps, passes or equipment and provides a wafer with improved die break strength and reduced debris by avoiding the debris plume caused by laser machining.